The present invention relates to the detection of explosives hidden in packages, particularly small amounts of modern, highly-explosive, nitrogen-based plastic explosives hidden in airline bags.
The detection of explosive devices hidden in airline baggage is a significant problem, particularly in view of the development of modern plastic explosives which can be formed into various innocent-appearing shapes and which are sufficiently powerful that small quantities can destroy an aircraft in flight. In general, three different screening approaches for the detection of hidden explosives are known and employed to various degrees at certain airports.
The first of these approaches is conventional X-ray imaging. Mere X-ray imaging however is of limited effectiveness, particularly since explosives need not be formed into any particular shape.
The second approach is the use of a so-called vapor sniffer which collects vapors emanating from luggage and analyzes them for the presence of molecules of explosive materials. While such devices are relatively sensitive, they nevertheless cannot detect explosives which are sealed within containers so as to prevent the escape of sufficient vapor quantities for detection.
The third approach involves the detection of nitrogen by means of thermal neutron interrogation. Nitrogen is a component of virtually every practical known high explosive. Thermal neutron interrogation involves exposing baggage to a "sea" of thermal neutrons (or "slow" neutrons having an energy in the order of 0.025 eV). Thermal neutrons combine with the nuclei of nitrogen-14 atoms to produce an energetic form of nitrogen-15 isotope. The energetic nitrogen-15 isotope immediately decays to its ground state, emitting characteristic 10.8 MeV gamma rays in the process. The 10.8 MeV gamma rays are detected as indicator of the presence of nitrogen in the package.
There are, however, a number of problems with such detectors employing thermal neutrons. A typical neutron source is radioactive californium-252 which emits energetic neutrons that are then slowed to thermal energies for reaction with nitrogen-14 nuclei. The use of such a radioactive neutron source introduces logistical problems related to handling and radiation shielding.
Other disadvantages of such systems include a relatively high false-positive rate coupled with the inability to effectively detect small quantities of explosives, such as quantities less than about one or two pounds. In particular, there are a number of materials other than explosives which contain nitrogen, such as wool and leather. If the threshold level of a system employing thermal neutron interrogation is adjusted so as to detect small quantities of nitrogen, then a high false-positive rate results due to the presence of innocent nitrogen-containing materials, leading to the necessity of searching an excessive number of packages by hand, negating the practical effectiveness of the system. If, on the other hand, the threshold level is set high to avoid false-positives, then the likelihood that actual explosives will escape detection is increased, again negating the effectiveness of the system.